In following the teachings of the prior art, e.g., Spurrell in U.S. Pat. No. 3,770,578 or Goyette in U.S. Pat. No. 2,981,175 according to which hot or cold air is applied to individual longitudinal sections of a calender roll from a hot air plenum or a cold air plenum in response to variations from the normal thickness of a paper web passing through the calendar, it is now recognized from actual practice that the temperature of the air in the walls of the hot or cold air plenums and associated air supply systems approaches room temperature during periods of little demand for air. Hence, when a caliper correction is required, the nozzles responsible for delivering air at a control temperature discharge air for at least a measurable time period at an improper temperature until the hot or cold air supply system reaches equilibrium with the temperature of air passing through it. Such a mode of operation prevents quick correction of the web thickness and thus achievement of the quality potential of the web product that is possible.
In past practice, control of the caliper by changing the diameter of the calendar roll was affected by impinging hot or cold air on one circumference of the roll and varying by velocity of air impingement to achieve the effect desired. Frequently, however, the roll diameter at each side of the circumference being corrected was unduly disturbed. The remedy was to impinge either cool or hot air, as the case required, on the roll at the sides of the circumference being corrected. The velocity of the air was varied according to the judgment of the operator. According to this earlier practice, air was not applied to the roll except during circumference correction.
Objects of the invention are to provide an apparatus that will enable more precise control in the processing of webs especially webs of paper to desired thicknesses or caliper; to provide apparatuses more responsive to caliper-sensing mechanism; to provide a continuous flow of air at a standard temperature to maintain a uniform circumference along the calender roll and eliminate the need for compensating or correcting air application to the roll at both sides of a target circumference being corrected; and to adopt a method which will require merely changing one variable, i.e., temperature of the air issuing from the nozzles to effect proper circumference correction of the roll along its length for uniform web thickness.